Hydrocarbons such as propane, butane, and other alkanes and alkenes are in widespread use, both as fuels and as the precursors for many vital and necessary chemical compounds such as plastics, detergents, pharmaceuticals, etc. Currently the primary sources of these hydrocarbons are fossil fuels, such as natural gas, from which they can be isolated. Such natural sources are, however, only available in limited supply, and retrieval and processing can have undesirable environmental impacts. In addition, the availability and pricing of such fossil fuels is greatly impacted by unpredictable political and social events.
Alternatives sources of such hydrocarbons have been developed, including the so-called Fischer-Tropsch synthesis. One version of this synthesis involves a series of reactions that uses hydrogen gas (H2) to reduce carbon monoxide (CO) in the presence of a metal catalyst (such as cobalt-, nickel-, iron-, or ruthenium-based catalysts) to produce carbon-carbon bond containing alkanes (CnH(2n+2)) and water (H2O). CO can be derived from a number of sources, including waste products of combustion of fossil fuels, natural gas, coal, and biomass. However, the efficiency of this synthesis is relatively low-generally only 25% to 50%. In addition, the process requires elevated temperatures and pressures in order to produce the desired range of alkane products, and careful control of reaction conditions and contaminants in the raw materials is necessary to avoid deactivation of the metal catalyst. As a result, while the Fischer-Tropsch synthesis has been developed for large scale production, it is not in widespread use, due in part to the high costs of reactor construction, operation, and maintenance.
In some instances it has been possible to replace such processes with the use of naturally occurring catalysts (e.g., enzymes) that can catalyze a wide variety of complex reactions at ambient temperatures and pressures, often at high efficiency. For example, monooxygenase enzymes have been used to catalyze oxidation of various hydrocarbons, such as ethylene, in the presence of oxygen to produce, for example, ethanol or ethylene oxide.
In another approach chemoautotrophic microorganisms, which are able to utilize inorganic carbon, are grown in a bioreactor using carbon dioxide (CO2) as a carbon source. Growth of these bacteria provides a biomass that may then be dried and harvested for useful components, for instance lipids and fats can be extracted from dried biomass using solvents and after additional processing may subsequently be used as fuels. Reactor designs are, however, complex in order to accommodate the environmental requirements for chemoautotrophic bacteria. In addition, while this approach does provide reduction of inorganic carbon under relatively mild conditions the resulting product is a highly complex mixture of biomolecules that requires extensive processing in order to isolate useful compounds.
Thus, there is a need for a system and method that can provide reduction of inorganic carbon, such as CO and CO2, to generate hydrocarbons such as alkanes and alkenes under mild conditions.